It has previously been proposed to drive alternators directly from an output shaft of an internal combustion engine of the vehicular type; such engines operate under widely varying conditions of speed, and the alternator must provide its rated output under widely varying conditions of speed. Consequently, a fan blade attached to the alternator should provide for adequate cooling thereof under the widely varying speed and loading conditions. Yet, if the engine operates at high speed and the alternator is only lightly loaded, a fan wheel coupled directly to the alternator will rotate at excessively high speed, causing losses, that is, unnecessarily loading the engine, and, additionally, increasing the overall noise level of the vehicle. It has been proposed--see the referenced U.S. Pat. No. 4,437,846, RAGALY--to provide a friction coupling, between the rotor shaft of the alternator, which is directly coupled to the drive shaft thereof, and the hub of a fan wheel, which is so arranged that, if the speed of the alternator increases, slippage between the hub of the fan wheel and the alternator shaft will be permitted, so that the fan wheel or ventilator will not be driven at a speed in excess of a load limit thereon, although the shaft may rotate at higher speeds due to higher speed operation of the engine. In one such construction, two ball bearings are located on the shaft of the alternator, the inner races thereof being fitted on the shaft of the alternator, whereas the outer races are fitted into a bore of a hub forming an integral casting with the fan wheel. The outer race of the ball bearing which is closer to the end shield or end bell of the alternator is press-fitted in the hub; the outer race of the ball bearing which is closest to the drive pulley of the alternator is so arranged that it can slip in the bore of the hub, and thus accomodate differences in speed between the drive shaft and the ventilating wheel. It has been found that the customary casting material used for the fan blade wears under the slipping conditions and, after some operating time, may become loose in the hub, so that the torques transmitted by this coupling may vary from design values.